Such pressure-relief valves, also known as safety valves, are used there where a hydraulic system, particularly a system of hydraulic struts drops in underground coal mining, can be damaged by excessive pressure. These hydraulic struts are arranged either as individual struts or integrated in a shield-type support. Because of the constant danger, there are official requirements for providing pressure-relief valves not only such shield-type support systems, but also for the individual hydraulic struts, in order to prevent permanent damage or even destruction endangering the miners, in cases of overload. DE-OS 28 30 891 describes a pressure-relief valve wherein excessive pressure in the hydraulic system is reduced by means of a valve spring braced between the locking screw and the valve piston. On the valve spring cap or the valve spring retainer a conically or spherically shaped valve-closing body is provided which is lifted off the valve seat in case of overload. On the piston a damping cylinder is provided, which limits the flow passage opening. However, such valves do not afford the locking safety required for the use as pressure-relief valves or safety valves in mining. Furthermore an accurate positioning particularly of the spring is very difficult, which means that all of the requirement for a safe response of such a mining safety valve are not fulfilled.
DE-OS 33 14 837 described another pressure-relief valve wherein the valve spring is arranged in the valve housing so that it presses on the valve spring cap and thereby on the valve piston and this way influences the opening of the valve corresponding to the setting. The valve piston is slidably guided on a piston-pin bore formed in the guide, whereby the necessary sealing is provided by an O-shaped annular seal located in a groove. Each time the O-ring has to be completely overridden by the radial bores of the valve piston, before the pressure medium can exit in case of overload due to this excessive load, such O-ring valves have a short operational life. A further drawback of these known pressure-relief valves are the low throughflow values of 40 to 60 (maximum 100) liters per minute. This is not satisfactory for the required safety and quick response of such valves. Besides, it is disadvantageous that the valve springs involved have to use wire of considerable strength and have considerable coil diameter in order to counteract the pressures; the spring size requires correspondingly large overall valve dimensions. The large the throughflow, the bigger the valve springs, and thereby the entire valve housing, have to be. In this case, particularly in underground mining, it is not possible to use very large valves already because of the large dimensions not to mention that the required cross sections for the evacuation of the pressure medium would not be available.